[libcxx] Enable threads on LLVM-libc (#224436)

Now that LLVM-libc's use of atomics has been fixed in #224162, #224160,
and #224164 this PR removes the carveout for threads in libc++ when
building on LLVM-libc.

Assisted-by: Automated tooling, human reviewed
GitOrigin-RevId: c495e51745bd34c74b1748a2b03ff8fa1edaa1c3